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At this point, you should have the GNU tools configured, built, and installed on your system. In this chapter, we present a simple example of using the GNU tools in an AVR project. After reading this chapter, you should have a better feel as to how the tools are used and how a Makefile can be configured.
This project will use the pulse-width modulator (PWM) to ramp an LED on and off every two seconds. An AT90S2313 processor will be used as the controller. The circuit for this demonstration is shown in the schematic diagram. If you have a development kit, you should be able to use it, rather than build the circuit, for this project.
Schematic of circuit for demo project
The source code is given in demo.c. For the sake of this example, create a file called demo.c containing this source code. Some of the more important parts of the code are:
iocompat.h tries to abstract between all this differences using some preprocessor #ifdef statements, so the actual program itself can operate on a common set of symbolic names. The macros defined by that file are:OCR the name of the OCR register used to control the PWM (usually either OCR1 or OCR1A)DDROC the name of the DDR (data direction register) for the OC outputOC1 the pin number of the OC1[A] output within its portTIMER1_TOP the TOP value of the timer used for the PWM (1023 for 10-bit PWMs, 255 for devices that can only handle an 8-bit PWM)TIMER1_PWM_INIT the initialization bits to be set into control register 1A in order to setup 10-bit (or 8-bit) phase and frequency correct PWM modeTIMER1_CLOCKSOURCE the clock bits to set in the respective control register to start the PWM timer; usually the timer runs at full CPU clock for 10-bit PWMs, while it runs on a prescaled clock for 8-bit PWMsPWM is being used in 10-bit mode, so we need a 16-bit variable to remember the current value.PWM.PWM register. Since we are in an interrupt routine, it is safe to use a 16-bit assignment to the register. Outside of an interrupt, the assignment should only be performed with interrupts disabled if there's a chance that an interrupt routine could also access this register (or another register that uses TEMP), see the appropriate FAQ entry.PWM and enables interrupts.sleep_mode() puts the processor on sleep until the next interrupt, to conserve power. Of course, that probably won't be noticable as we are still driving a LED, it is merely mentioned here to demonstrate the basic principle./* * ---------------------------------------------------------------------------- * "THE BEER-WARE LICENSE" (Revision 42): * <joerg@FreeBSD.ORG> wrote this file. As long as you retain this notice you * can do whatever you want with this stuff. If we meet some day, and you think * this stuff is worth it, you can buy me a beer in return. Joerg Wunsch * ---------------------------------------------------------------------------- * * Simple AVR demonstration. Controls a LED that can be directly * connected from OC1/OC1A to GND. The brightness of the LED is * controlled with the PWM. After each period of the PWM, the PWM * value is either incremented or decremented, that's all. * * $Id: demo.c 1637 2008-03-17 21:49:41Z joerg_wunsch $ */ #include <inttypes.h> #include <avr/io.h> #include <avr/interrupt.h> #include <avr/sleep.h> #include "iocompat.h" /* Note [1] */ enum { UP, DOWN }; ISR (TIMER1_OVF_vect) /* Note [2] */ { static uint16_t pwm; /* Note [3] */ static uint8_t direction; switch (direction) /* Note [4] */ { case UP: if (++pwm == TIMER1_TOP) direction = DOWN; break; case DOWN: if (--pwm == 0) direction = UP; break; } OCR = pwm; /* Note [5] */ } void ioinit (void) /* Note [6] */ { /* Timer 1 is 10-bit PWM (8-bit PWM on some ATtinys). */ TCCR1A = TIMER1_PWM_INIT; /* * Start timer 1. * * NB: TCCR1A and TCCR1B could actually be the same register, so * take care to not clobber it. */ TCCR1B |= TIMER1_CLOCKSOURCE; /* * Run any device-dependent timer 1 setup hook if present. */ #if defined(TIMER1_SETUP_HOOK) TIMER1_SETUP_HOOK(); #endif /* Set PWM value to 0. */ OCR = 0; /* Enable OC1 as output. */ DDROC = _BV (OC1); /* Enable timer 1 overflow interrupt. */ TIMSK = _BV (TOIE1); sei (); } int main (void) { ioinit (); /* loop forever, the interrupts are doing the rest */ for (;;) /* Note [7] */ sleep_mode(); return (0); }
This first thing that needs to be done is compile the source. When compiling, the compiler needs to know the processor type so the -mmcu option is specified. The -Os option will tell the compiler to optimize the code for efficient space usage (at the possible expense of code execution speed). The -g is used to embed debug info. The debug info is useful for disassemblies and doesn't end up in the .hex files, so I usually specify it. Finally, the -c tells the compiler to compile and stop -- don't link. This demo is small enough that we could compile and link in one step. However, real-world projects will have several modules and will typically need to break up the building of the project into several compiles and one link.
$ avr-gcc -g -Os -mmcu=atmega8 -c demo.c
The compilation will create a demo.o file. Next we link it into a binary called demo.elf.
$ avr-gcc -g -mmcu=atmega8 -o demo.elf demo.o
It is important to specify the MCU type when linking. The compiler uses the -mmcu option to choose start-up files and run-time libraries that get linked together. If this option isn't specified, the compiler defaults to the 8515 processor environment, which is most certainly what you didn't want.
Now we have a binary file. Can we do anything useful with it (besides put it into the processor?) The GNU Binutils suite is made up of many useful tools for manipulating object files that get generated. One tool is avr-objdump, which takes information from the object file and displays it in many useful ways. Typing the command by itself will cause it to list out its options.
For instance, to get a feel of the application's size, the -h option can be used. The output of this option shows how much space is used in each of the sections (the .stab and .stabstr sections hold the debugging information and won't make it into the ROM file).
An even more useful option is -S. This option disassembles the binary file and intersperses the source code in the output! This method is much better, in my opinion, than using the -S with the compiler because this listing includes routines from the libraries and the vector table contents. Also, all the "fix-ups" have been satisfied. In other words, the listing generated by this option reflects the actual code that the processor will run.
$ avr-objdump -h -S demo.elf > demo.lst
Here's the output as saved in the demo.lst file:
demo.elf: file format elf32-avr
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 000000ca 00000000 00000000 00000074 2**1
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .bss 00000003 00800060 00800060 0000013e 2**0
ALLOC
2 .debug_aranges 00000020 00000000 00000000 0000013e 2**0
CONTENTS, READONLY, DEBUGGING
3 .debug_pubnames 00000035 00000000 00000000 0000015e 2**0
CONTENTS, READONLY, DEBUGGING
4 .debug_info 00000108 00000000 00000000 00000193 2**0
CONTENTS, READONLY, DEBUGGING
5 .debug_abbrev 000000cf 00000000 00000000 0000029b 2**0
CONTENTS, READONLY, DEBUGGING
6 .debug_line 00000165 00000000 00000000 0000036a 2**0
CONTENTS, READONLY, DEBUGGING
7 .debug_frame 00000040 00000000 00000000 000004d0 2**2
CONTENTS, READONLY, DEBUGGING
8 .debug_str 000000cc 00000000 00000000 00000510 2**0
CONTENTS, READONLY, DEBUGGING
9 .debug_pubtypes 0000002b 00000000 00000000 000005dc 2**0
CONTENTS, READONLY, DEBUGGING
Disassembly of section .text:
00000000 <__ctors_end>:
0: 10 e0 ldi r17, 0x00 ; 0
2: a0 e6 ldi r26, 0x60 ; 96
4: b0 e0 ldi r27, 0x00 ; 0
6: 01 c0 rjmp .+2 ; 0xa <.do_clear_bss_start>
00000008 <.do_clear_bss_loop>:
8: 1d 92 st X+, r1
0000000a <.do_clear_bss_start>:
a: a3 36 cpi r26, 0x63 ; 99
c: b1 07 cpc r27, r17
e: e1 f7 brne .-8 ; 0x8 <.do_clear_bss_loop>
00000010 <__vector_8>:
#include "iocompat.h" /* Note [1] */
enum { UP, DOWN };
ISR (TIMER1_OVF_vect) /* Note [2] */
{
10: 1f 92 push r1
12: 0f 92 push r0
14: 0f b6 in r0, 0x3f ; 63
16: 0f 92 push r0
18: 11 24 eor r1, r1
1a: 2f 93 push r18
1c: 8f 93 push r24
1e: 9f 93 push r25
static uint16_t pwm; /* Note [3] */
static uint8_t direction;
switch (direction) /* Note [4] */
20: 80 91 60 00 lds r24, 0x0060
24: 88 23 and r24, r24
26: b9 f4 brne .+46 ; 0x56 <__SREG__+0x17>
{
case UP:
if (++pwm == TIMER1_TOP)
28: 80 91 61 00 lds r24, 0x0061
2c: 90 91 62 00 lds r25, 0x0062
30: 01 96 adiw r24, 0x01 ; 1
32: 90 93 62 00 sts 0x0062, r25
36: 80 93 61 00 sts 0x0061, r24
3a: 23 e0 ldi r18, 0x03 ; 3
3c: 8f 3f cpi r24, 0xFF ; 255
3e: 92 07 cpc r25, r18
40: f9 f0 breq .+62 ; 0x80 <__SREG__+0x41>
if (--pwm == 0)
direction = UP;
break;
}
OCR = pwm; /* Note [5] */
42: 9b bd out 0x2b, r25 ; 43
44: 8a bd out 0x2a, r24 ; 42
}
46: 9f 91 pop r25
48: 8f 91 pop r24
4a: 2f 91 pop r18
4c: 0f 90 pop r0
4e: 0f be out 0x3f, r0 ; 63
50: 0f 90 pop r0
52: 1f 90 pop r1
54: 18 95 reti
ISR (TIMER1_OVF_vect) /* Note [2] */
{
static uint16_t pwm; /* Note [3] */
static uint8_t direction;
switch (direction) /* Note [4] */
56: 81 30 cpi r24, 0x01 ; 1
58: 29 f0 breq .+10 ; 0x64 <__SREG__+0x25>
5a: 80 91 61 00 lds r24, 0x0061
5e: 90 91 62 00 lds r25, 0x0062
62: ef cf rjmp .-34 ; 0x42 <__SREG__+0x3>
if (++pwm == TIMER1_TOP)
direction = DOWN;
break;
case DOWN:
if (--pwm == 0)
64: 80 91 61 00 lds r24, 0x0061
68: 90 91 62 00 lds r25, 0x0062
6c: 01 97 sbiw r24, 0x01 ; 1
6e: 90 93 62 00 sts 0x0062, r25
72: 80 93 61 00 sts 0x0061, r24
76: 00 97 sbiw r24, 0x00 ; 0
78: 21 f7 brne .-56 ; 0x42 <__SREG__+0x3>
direction = UP;
7a: 10 92 60 00 sts 0x0060, r1
7e: e1 cf rjmp .-62 ; 0x42 <__SREG__+0x3>
switch (direction) /* Note [4] */
{
case UP:
if (++pwm == TIMER1_TOP)
direction = DOWN;
80: 21 e0 ldi r18, 0x01 ; 1
82: 20 93 60 00 sts 0x0060, r18
86: dd cf rjmp .-70 ; 0x42 <__SREG__+0x3>
00000088 <ioinit>:
void
ioinit (void) /* Note [6] */
{
/* Timer 1 is 10-bit PWM (8-bit PWM on some ATtinys). */
TCCR1A = TIMER1_PWM_INIT;
88: 83 e8 ldi r24, 0x83 ; 131
8a: 8f bd out 0x2f, r24 ; 47
* Start timer 1.
*
* NB: TCCR1A and TCCR1B could actually be the same register, so
* take care to not clobber it.
*/
TCCR1B |= TIMER1_CLOCKSOURCE;
8c: 8e b5 in r24, 0x2e ; 46
8e: 81 60 ori r24, 0x01 ; 1
90: 8e bd out 0x2e, r24 ; 46
#if defined(TIMER1_SETUP_HOOK)
TIMER1_SETUP_HOOK();
#endif
/* Set PWM value to 0. */
OCR = 0;
92: 1b bc out 0x2b, r1 ; 43
94: 1a bc out 0x2a, r1 ; 42
/* Enable OC1 as output. */
DDROC = _BV (OC1);
96: 82 e0 ldi r24, 0x02 ; 2
98: 87 bb out 0x17, r24 ; 23
/* Enable timer 1 overflow interrupt. */
TIMSK = _BV (TOIE1);
9a: 84 e0 ldi r24, 0x04 ; 4
9c: 89 bf out 0x39, r24 ; 57
sei ();
9e: 78 94 sei
}
a0: 08 95 ret
000000a2 <main>:
void
ioinit (void) /* Note [6] */
{
/* Timer 1 is 10-bit PWM (8-bit PWM on some ATtinys). */
TCCR1A = TIMER1_PWM_INIT;
a2: 83 e8 ldi r24, 0x83 ; 131
a4: 8f bd out 0x2f, r24 ; 47
* Start timer 1.
*
* NB: TCCR1A and TCCR1B could actually be the same register, so
* take care to not clobber it.
*/
TCCR1B |= TIMER1_CLOCKSOURCE;
a6: 8e b5 in r24, 0x2e ; 46
a8: 81 60 ori r24, 0x01 ; 1
aa: 8e bd out 0x2e, r24 ; 46
#if defined(TIMER1_SETUP_HOOK)
TIMER1_SETUP_HOOK();
#endif
/* Set PWM value to 0. */
OCR = 0;
ac: 1b bc out 0x2b, r1 ; 43
ae: 1a bc out 0x2a, r1 ; 42
/* Enable OC1 as output. */
DDROC = _BV (OC1);
b0: 82 e0 ldi r24, 0x02 ; 2
b2: 87 bb out 0x17, r24 ; 23
/* Enable timer 1 overflow interrupt. */
TIMSK = _BV (TOIE1);
b4: 84 e0 ldi r24, 0x04 ; 4
b6: 89 bf out 0x39, r24 ; 57
sei ();
b8: 78 94 sei
ioinit ();
/* loop forever, the interrupts are doing the rest */
for (;;) /* Note [7] */
sleep_mode();
ba: 85 b7 in r24, 0x35 ; 53
bc: 80 68 ori r24, 0x80 ; 128
be: 85 bf out 0x35, r24 ; 53
c0: 88 95 sleep
c2: 85 b7 in r24, 0x35 ; 53
c4: 8f 77 andi r24, 0x7F ; 127
c6: 85 bf out 0x35, r24 ; 53
c8: f8 cf rjmp .-16 ; 0xba <main+0x18>
avr-objdump is very useful, but sometimes it's necessary to see information about the link that can only be generated by the linker. A map file contains this information. A map file is useful for monitoring the sizes of your code and data. It also shows where modules are loaded and which modules were loaded from libraries. It is yet another view of your application. To get a map file, I usually add -Wl,-Map,demo.map to my link command. Relink the application using the following command to generate demo.map (a portion of which is shown below).
$ avr-gcc -g -mmcu=atmega8 -Wl,-Map,demo.map -o demo.elf demo.o
Some points of interest in the demo.map file are:
.rela.plt
*(.rela.plt)
.text 0x0000000000000000 0xca
*(.vectors)
*(.vectors)
*(.progmem.gcc*)
*(.progmem*)
0x0000000000000000 . = ALIGN (0x2)
0x0000000000000000 __trampolines_start = .
*(.trampolines)
.trampolines 0x0000000000000000 0x0 linker stubs
*(.trampolines*)
0x0000000000000000 __trampolines_end = .
*(.jumptables)
*(.jumptables*)
*(.lowtext)
*(.lowtext*)
0x0000000000000000 __ctors_start = .
The .text segment (where program instructions are stored) starts at location 0x0.
*(.fini2)
*(.fini2)
*(.fini1)
*(.fini1)
*(.fini0)
*(.fini0)
0x00000000000000ca _etext = .
.data 0x0000000000800060 0x0 load address 0x00000000000000ca
0x0000000000800060 PROVIDE (__data_start, .)
*(.data)
.data 0x0000000000800060 0x0 demo.o
.data 0x0000000000800060 0x0 /home/tools/hudson/workspace/avr8-gnu-toolchain/avr8-gnu-toolchain-linux_x86_64/lib/gcc/avr/4.5.1/avr4/libgcc.a(_clear_bss.o)
*(.data*)
*(.rodata)
*(.rodata*)
*(.gnu.linkonce.d*)
0x0000000000800060 . = ALIGN (0x2)
0x0000000000800060 _edata = .
0x0000000000800060 PROVIDE (__data_end, .)
.bss 0x0000000000800060 0x3
0x0000000000800060 PROVIDE (__bss_start, .)
*(.bss)
.bss 0x0000000000800060 0x3 demo.o
.bss 0x0000000000800063 0x0 /home/tools/hudson/workspace/avr8-gnu-toolchain/avr8-gnu-toolchain-linux_x86_64/lib/gcc/avr/4.5.1/avr4/libgcc.a(_clear_bss.o)
*(.bss*)
*(COMMON)
0x0000000000800063 PROVIDE (__bss_end, .)
0x00000000000000ca __data_load_start = LOADADDR (.data)
0x00000000000000ca __data_load_end = (__data_load_start + SIZEOF (.data))
.noinit 0x0000000000800063 0x0
0x0000000000800063 PROVIDE (__noinit_start, .)
*(.noinit*)
0x0000000000800063 PROVIDE (__noinit_end, .)
0x0000000000800063 _end = .
0x0000000000800063 PROVIDE (__heap_start, .)
.eeprom 0x0000000000810000 0x0
*(.eeprom*)
0x0000000000810000 __eeprom_end = .
The last address in the .text segment is location 0x114 ( denoted by _etext ), so the instructions use up 276 bytes of FLASH.
The .data segment (where initialized static variables are stored) starts at location 0x60, which is the first address after the register bank on an ATmega8 processor.
The next available address in the .data segment is also location 0x60, so the application has no initialized data.
The .bss segment (where uninitialized data is stored) starts at location 0x60.
The next available address in the .bss segment is location 0x63, so the application uses 3 bytes of uninitialized data.
The .eeprom segment (where EEPROM variables are stored) starts at location 0x0.
The next available address in the .eeprom segment is also location 0x0, so there aren't any EEPROM variables.
We have a binary of the application, but how do we get it into the processor? Most (if not all) programmers will not accept a GNU executable as an input file, so we need to do a little more processing. The next step is to extract portions of the binary and save the information into .hex files. The GNU utility that does this is called avr-objcopy.
The ROM contents can be pulled from our project's binary and put into the file demo.hex using the following command:
$ avr-objcopy -j .text -j .data -O ihex demo.elf demo.hex
The resulting demo.hex file contains:
:1000000010E0A0E6B0E001C01D92A336B107E1F711 :100010001F920F920FB60F9211242F938F939F93DD :10002000809160008823B9F4809161009091620012 :100030000196909362008093610023E08F3F9207C6 :10004000F9F09BBD8ABD9F918F912F910F900FBEAC :100050000F901F901895813029F080916100909148 :100060006200EFCF809161009091620001979093C0 :10007000620080936100009721F710926000E1CF49 :1000800021E020936000DDCF83E88FBD8EB58160D5 :100090008EBD1BBC1ABC82E087BB84E089BF78940C :1000A000089583E88FBD8EB581608EBD1BBC1ABCE0 :1000B00082E087BB84E089BF789485B7806885BF7C :0A00C000889585B78F7785BFF8CFCC :00000001FF
The -j option indicates that we want the information from the .text and .data segment extracted. If we specify the EEPROM segment, we can generate a .hex file that can be used to program the EEPROM:
$ avr-objcopy -j .eeprom --change-section-lma .eeprom=0 -O ihex demo.elf demo_eeprom.hex
There is no demo_eeprom.hex file written, as that file would be empty.
Starting with version 2.17 of the GNU binutils, the avr-objcopy command that used to generate the empty EEPROM files now aborts because of the empty input section .eeprom, so these empty files are not generated. It also signals an error to the Makefile which will be caught there, and makes it print a message about the empty file not being generated.
Rather than type these commands over and over, they can all be placed in a make file. To build the demo project using make, save the following in a file called Makefile.
Makefile can only be used as input for the GNU version of make.PRG = demo OBJ = demo.o #MCU_TARGET = at90s2313 #MCU_TARGET = at90s2333 #MCU_TARGET = at90s4414 #MCU_TARGET = at90s4433 #MCU_TARGET = at90s4434 #MCU_TARGET = at90s8515 #MCU_TARGET = at90s8535 #MCU_TARGET = atmega128 #MCU_TARGET = atmega1280 #MCU_TARGET = atmega1281 #MCU_TARGET = atmega1284p #MCU_TARGET = atmega16 #MCU_TARGET = atmega163 #MCU_TARGET = atmega164p #MCU_TARGET = atmega165 #MCU_TARGET = atmega165p #MCU_TARGET = atmega168 #MCU_TARGET = atmega169 #MCU_TARGET = atmega169p #MCU_TARGET = atmega2560 #MCU_TARGET = atmega2561 #MCU_TARGET = atmega32 #MCU_TARGET = atmega324p #MCU_TARGET = atmega325 #MCU_TARGET = atmega3250 #MCU_TARGET = atmega329 #MCU_TARGET = atmega3290 #MCU_TARGET = atmega48 #MCU_TARGET = atmega64 #MCU_TARGET = atmega640 #MCU_TARGET = atmega644 #MCU_TARGET = atmega644p #MCU_TARGET = atmega645 #MCU_TARGET = atmega6450 #MCU_TARGET = atmega649 #MCU_TARGET = atmega6490 MCU_TARGET = atmega8 #MCU_TARGET = atmega8515 #MCU_TARGET = atmega8535 #MCU_TARGET = atmega88 #MCU_TARGET = attiny2313 #MCU_TARGET = attiny24 #MCU_TARGET = attiny25 #MCU_TARGET = attiny26 #MCU_TARGET = attiny261 #MCU_TARGET = attiny44 #MCU_TARGET = attiny45 #MCU_TARGET = attiny461 #MCU_TARGET = attiny84 #MCU_TARGET = attiny85 #MCU_TARGET = attiny861 OPTIMIZE = -O2 DEFS = LIBS = # You should not have to change anything below here. CC = avr-gcc # Override is only needed by avr-lib build system. override CFLAGS = -g -Wall $(OPTIMIZE) -mmcu=$(MCU_TARGET) $(DEFS) override LDFLAGS = -Wl,-Map,$(PRG).map OBJCOPY = avr-objcopy OBJDUMP = avr-objdump all: $(PRG).elf lst text eeprom $(PRG).elf: $(OBJ) $(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LIBS) # dependency: demo.o: demo.c iocompat.h clean: rm -rf *.o $(PRG).elf *.eps *.png *.pdf *.bak rm -rf *.lst *.map $(EXTRA_CLEAN_FILES) lst: $(PRG).lst %.lst: %.elf $(OBJDUMP) -h -S $< > $@ # Rules for building the .text rom images text: hex bin srec hex: $(PRG).hex bin: $(PRG).bin srec: $(PRG).srec %.hex: %.elf $(OBJCOPY) -j .text -j .data -O ihex $< $@ %.srec: %.elf $(OBJCOPY) -j .text -j .data -O srec $< $@ %.bin: %.elf $(OBJCOPY) -j .text -j .data -O binary $< $@ # Rules for building the .eeprom rom images eeprom: ehex ebin esrec ehex: $(PRG)_eeprom.hex ebin: $(PRG)_eeprom.bin esrec: $(PRG)_eeprom.srec %_eeprom.hex: %.elf $(OBJCOPY) -j .eeprom --change-section-lma .eeprom=0 -O ihex $< $@ \ || { echo empty $@ not generated; exit 0; } %_eeprom.srec: %.elf $(OBJCOPY) -j .eeprom --change-section-lma .eeprom=0 -O srec $< $@ \ || { echo empty $@ not generated; exit 0; } %_eeprom.bin: %.elf $(OBJCOPY) -j .eeprom --change-section-lma .eeprom=0 -O binary $< $@ \ || { echo empty $@ not generated; exit 0; } # Every thing below here is used by avr-libc's build system and can be ignored # by the casual user. FIG2DEV = fig2dev EXTRA_CLEAN_FILES = *.hex *.bin *.srec dox: eps png pdf eps: $(PRG).eps png: $(PRG).png pdf: $(PRG).pdf %.eps: %.fig $(FIG2DEV) -L eps $< $@ %.pdf: %.fig $(FIG2DEV) -L pdf $< $@ %.png: %.fig $(FIG2DEV) -L png $< $@